Every four years the city-states of Ancient Greece used to call a truce and head to Olympia for an athletics competition in worship of Zeus. The tradition began in 776 BC and continued until suppressed by Roman Emperor Theodosius I in 394 AD as too pagan.
Today, Olympia is one of Greece's most visited historic sites and has the Museum of the History of the Ancient Olympic Games.
This small museum has got plenty of information about the origins of the Olympic Games but is small enough that the kids won't get bored. You can take it all in in about half an hour.
It's located in a neo-classical building a little away from the archeological site of the ancient games but worth finding for its information about the ancient games held from 776BC to 393 AD. Did you know that races with armor were introduced in 520BC, and the race for chariots drawn by four foals in 380BC? Or that the winners in the Ancient Olympics were granted special life-long privileges like having all their food paid for by the city?